Magnetic recording media are widely used as, for example, recording tapes, video tapes, computer tapes, disks. Recently higher-density recording has been required for the magnetic recording media, and, as a result, recording wavelengths have been shortened. Recording system has been examined over a digital system as well as an analog system. Magnetic recording media using a metal thin layer as the magnetic layer have been proposed to satisfy the requirements of high-density recording.
Various methods have been conventionally proposed to improve the electromagnetic characteristics of the coated type magnetic recording media. For example, the improvements of magnetic characteristics of ferromagnetic powder or smooth of surface have been proposed. However, these proposals have been still not sufficient for the requirements of higher-density recording.
Further, recording wavelengths are likely to be shortened with higher-density recording in recent years. When the thickness of the magnetic layer of the magnetic recording media is thick, there are caused problems with regard to lowering in output, self-demagnetization loss during recording and thickness loss during reproduction.
Accordingly, it is desirable to thin the thickness of the magnetic layer. However, when the thickness of the magnetic layer is thinned, the surface of the magnetic layer is likely to be affected by the surface roughness of the nonmagnetic support, and electromagnetic characteristics are likely to be deteriorated. Further, when excellent electromagnetic characteristics are to be ensured, there is caused a problem that good running durability cannot be obtained.
Various experiments have been made to improve the dispersibility of ferromagnetic particle by improving the binder as well as to improve the mechanical strength of the magnetic layer, thereby solving the above-described problems.
For example,
a) JP-A-1-236290 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses that a squareness ratio and abrasion resistance can be improved by using polyvinyl acetal resins having a degree of acetoacetalization of 85% by weight or more; PA0 b) JP-A-2-220221 discloses that polyvinyl acetal resins comprising a unit composition having a hydrophilic group are used to improve the squareness ratio and the S/N ratio; PA0 c) JP-A-60-34166 discloses polyvinyl acetal resins having a --COOM group; and PA0 d) JP-B-58-41565 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses polyurethane resins having an --SO.sub.3 M group. PA0 (1) a magnetic recording medium comprising a nonmagnetic support having on at least one side thereof a magnetic layer comprising a ferromagnetic particle and a binder, wherein the binder comprises at least a polyvinyl acetal resin having a polar group and a polyurethane resin having a polar group; or PA0 (2) a magnetic recording medium comprising (a) a nonmagnetic support having on at least one side thereof at least one of a lower magnetic layer comprising a ferromagnetic particle and a binder and a nonmagnetic layer comprising a nonmagnetic particle and a binder and (b) an upper magnetic layer comprising a ferromagnetic particle and a binder provided thereon, wherein at least the binder contained in the upper magnetic layer comprises a polyvinyl acetal resin having a polar group and a polyurethane resin having a polar group. PA0 1) Weight Average Molecular Weight (Mw): 10,000 to 100,000; PA0 2) Weight Average Molecular Weight (Mw): 10,000 to 100,000;
However, it has been found that when a hydrophilic group is introduced into the polyvinyl acetal resins, the dispersibility can be improved, but the degree of the improvement it still insufficient, and the smoothness of the magnetic layer and the electromagnetic characteristics are also insufficient.
Further, the polyvinyl acetal resins have a higher Tg (glass transition temperature) and provide a higher film strength than vinyl chloride binders, but there are problems that the running durability thereof is insufficient, and clogging is caused.
The polyvinyl acetal resins have been used in combination with polyurethane resins to solve the problem with regard to clogging. However, these resins are not sufficiently compatible with each other, the polyurethane resins are likely to ooze out onto the surface of the magnetic layer, and the durability thereof is reduced. In addition, a new problem is caused that a trouble is likely to be caused by sticking. Namely, since the polyurethane resins oozed out onto the surface of the magnetic layer are soft, the resins are scraped by heads and sticked on the heads to thereby cause clogging. When a tape is wound around a reel, there is caused a problem that the tape in the wound-up state is likely to be stuck together by the polyurethane resin oozed out onto the surface of the magnetic layer.